1. Field of the Invention
This invention relates to a waveguide type optical device used for ultra high speed trunk fiber-optic transmission system, and more particularly relates to waveguide type polarization scrambler.
2. Description of Related Art
As the fiber-optic transmission system has been used practically, an advanced large capacity multi-functional transmission system including high speed generation of optical signals, wavelength multiplexing in the same optical transmission path, and switching and exchanging of optical transmission paths is desired to be developed.
In the current fiber-optic transmission system, the optical transmission technology employs the direct modulation method that involves direct control of injection current to a semiconductor laser or light emitting diode which are provided as a light source. The direct modulation is not suitable for high speed modulation of several GHz or higher because of relaxation oscillation, and cannot be used for high speed trunk fiber-optic transmission because of wavelength fluctuation.
A method in which an external modulation type modulator is used has been known as a method to solve the above-mentioned disadvantage, particularly, an optical waveguide type optical external modulator comprising an optical waveguide formed on a dielectric substrate which has optical effect has characteristics of small-size, high efficiency and high speed. Especially, optical external modulators which utilize dielectric material such as lithium niobate (LiNbO.sub.3) crystal have characteristic of small optical absorption, low optical loss, and high modulation efficiency for high electro-optic coefficient. Therefore many developments have addressed on the optical external modulator, and particularly, Mach-Zehnder interferometric type and directional coupling type external modulators have been proposed and partially used practically.
Recently, a trunk fiber-optic transmission system having a large capacity exceeding G bps has been realized by applying practically the external modulator which utilizes lithium niobate. On the other hand, rapid progress and practical application of optical amplifiers using Er.sup.+ doped optical fiber enables long distance fiber-optic transmission exceeding several thousands kilometers.
However, the long distance fiber-optic transmission using the optical amplifier involves a problem of reduced receiving sensitivity due to polarization fluctuation in a optical path of transmission system and polarization-dependent gain (PDG) in the optical amplifier. To solve the problem, a method in which the polarization of signal light in an optical path of transmission system is scramble has been known, and F. Heismann et. al. ("Electrooptic Polarization Scramblers for Optically Amplified Long-Haul Transmission Systems", IEEE Photonics Technology Letters, Vol. 6, No. 9, September, 1994, pp. 1156) reported the improvement of receiving sensitivity.
The waveguide scrambler is one of devices necessary for randomozed polarization state of the signal light. The waveguide type polarization scrambler which utilizes an optical waveguide is one type of polarization scramblers. A conventional waveguide type polarization scrambler with the inclination of an input linear polarized light to the substrate surface of 45 degrees (refer to as inclination angle hereinafter) can give the degree of polarization of D.O.P=0 only when the insertion loss polarization dependency of the optical waveguide is zero.
However, it is difficult to fabricate polarization independent optical waveguide 2 because of variation in actual fabrication process. In the case of inclination angle of 45 degrees, with increasing in polarization dependency of the optical waveguide 2 .DELTA.L=TM insertion loss-TE insertion loss (dB), D.O.P deteriorates.
The second problem is D.O.P deterioration due to polarization dependency caused by deviation of the optical axis between the optical axis after adjustment and after completion of curing the optical axis. Because possible deviation caused when an input fiber after adjustment of the optical axis is coupled to an input port with adhesives can cause the polarization dependency of coupling loss.